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Nanosecond pulse device for a 2 MeV Van de Graaff electron accelerator
NucLBAR
INSTRUMENTS
NANOSECOND
AND
METHODS
40 (1966)131-135;0 NORTH-HOLLAND
PULSE DEVICE FOR A 2 MeV VAN DE GRAAPF ELECTRON O.DEHOUST,H.BAIERL*
PUBLISHING
co.
ACCELERATOR
andE.HESSE+
Znstitut fiir Technische Elektronik der Technischen Hochschule, M&hen,
Germany
Received 15 September 1965
A pulse generator for a 2 MeV Van de Graaff accelerator is described which produces electron pulses with rise and decay times of lo-9 sec. The system is capable of delivering a maximum current of about 5 mA for a duration in the order of lo-8 sec.
For longer pulses the current pulse amplitude must be reduced, for example to 1.2 mA for a 2 x 10-7 set wide pulse, in order to obtain a good square-topped shape.
1. Introduction
homogeneous ionization can be produced by 2 MeV electrons is about 2.5 mm for silicon3).
Measurements of short carrier lifetimes in semiconductors and fast decay times in scintillation materials require a source of excitation with a switching time in the order of a nanosecond. Flashes of light with the necessary time resolution can be produced by use of a rotating mirror or a Kerr cell. The intensity of such flashes of light, however, is low and insufficient for measurement of very short lifetimes. Also the penetration depth of light is small when absorption takes place in base lattice which is necessary for a strong excitation. A more suitable excitation source is a pulsed electron beam. Wertheim and Augustyniaki) produced electron pulses with rise and decay times in the order of 5 x 10m9 set by pulsing a horizontal Van de Graaff accelerator (1 MV, 25 PA). Bauerlein2) obtained switching times shorter than 10m9 set by using a pulsed electron source in a 500 keV after-accelerator in Greinacher-circuit. In this paper a pulse device for a 2 MeV Van de Graaff accelerator is described which produces electron pulses with rise and decay times in the order of 10m9sec. This device may be located in every Van de Graaff accelerator without essential changes of the accelerator arrangement, provided there is enough room. In case higher energetic particles are used, the rate of change in the structure of the specimen under examination will increase, certainly, but no noticeable changes will occur within the normal time of experiment, provided a sufficiently low pulse repetition rate and beam current will be chosen. The advantage of applying particles of higher energy is that a homogeneous ionization may be produced even in thick samples. If the excitation is considered to be of sufficient homogeneity within 3 of the particle range, the maximum thickness in which a * Present address: Brown, Boveri and Cie AG, Marmheim, Germany. + Alexander von Humboldt Fellow. Present address: University of Chile, Santiago, Chile.
2. Apparatus There are three different methods for pulsing the electron beam of an electrostatic accelerator4) : 1. Pulsing the electron source by variation of the voltage between cathode and a control electrode. 2. Electric or magnetic deflection of the electron beam across an aperture before acceleration. 3. Sweeping the beam across an aperture after the electrons have passed the accelerating voltage. Furthermore, it is possible to combine two methods, for example methods 1 and 3. For the device described in this paper the first method was chosen, because it may be applied without important change in the system arrangement, allowing peak pulse currents much greater than the dc rating of the machinel,‘). In addition, there is no scattered electron and X-ray background during the off-period. The accelerator used is a 2 MeV Van de Graaff of the High Voltage Engineering Corporation in vertical arrangement. The accelerator tube allows a maximum beam current of 250 PA in dooperation. It contains a directly heated cathode enclosed in a cylindrical cap with an aperture for the electron beam. The beam current of the tube can be varied by changing the potential difference between cathode and cathode cap. The control characteristics of the accelerator tube for different accelerating voltages during normal Van de Graaff operation are shown in fig. 1. These characteristics were obtained with a first column resistor of 1430 megohm which is located between terminal plate (potential of the cathode cap) and first accelerator-tube electrode and which is equal to the second column resistor. Since the electrostatic field of the first accelerator electrode extends into the cathode region and withdraws emitted electrons the control characteristics of the accelerator tube depend on the magnitude of the first column resistor. The necessary variation of the bias 131
132
0. DEHOUST
CONTROL Fig. 1. Control characteristics
et al.
VOLTAGE
U, IN
VOLTS
of the accelerator tube for different accelerator voltages.
between cathode cap and cathode during measurement of the characteristics was performed by a potentiometer located for this purpose in the high voltage terminal. Hereto, the movement of a step switch operated by an insulated rod was coupled to the axis of the potentiometer by means of a transmission gearing. The tappets of the potentiometer have been removed so that the potentiometer axis may be turned in one direction as many times as wanted. Since a 360 rotation of the potentiometer does not coincide with an integral multiple of a switch step, the potentiometer adjustments after a full rotation are not the same as those before rotation. This makes clear why a L- 10% error mentioned in fig. 1 is caused. To cut off the electron beam for all accelerator voltages the cathode is biased +80 V with respect to the control electrode (grid), which is maintained at local ground potential (i.e. terminal voltage). Short negative square-topped pulses with an amplitude of more than - 80 V cancel the gate voltage and cause an electron current from the cathode to the target for the duration of the impulse. A pulse generator furnished with delay line supplies the negative pulses with rise and decay times less than 2 x low9 set (measured between 10% and 90% amplitude levels). A coaxial line with open end (Amphenol: type RG/U9B) is charged up to a potential U,. By a relay with mercury wetted contacts (Clare: type HG) a 50 ohm resistor matched to the characteristic impe-
dance of the line is put across the input end of the charged line. At this terminating resistor a pulse is produced with an amplitude of +U, and a duration of 22, where z is the time a wave needs for propagation along the length of the line5). The pulse duration can be varied by changing the length of the line. Three cables of different lengths placed on a cable reel in the terminal of the accelerator provided pulses with durations of 10, 50 or 200 nsec. Relay and terminating resistor are mounted in a coaxial housing in order to prevent pulse reflections. Fig. 2 shows details of the pulse generator. The relay of the mercury switch is energized by a transistorized monostable multivibrator. The monovibrator can be triggered by a single pulse induced by the switch S4 or by periodical pulses generated by an astable multivibrator. The pulse repetition rate may be altered by the potentiometer R4 in the range between 10 and 50 cps. It may be increased to 300 cps by insignificant changes of the multivibrator and by use of another Clare-relay (type HGS). The power supply unit provides the operating voltage for the transistorized multivibrators, the cut-off voltage applied between cathode and control electrode, and the negative voltage for charging the delay line. The charging voltage and, therefore, the amplitude of the pulse, generating the beam current, may be altered by the selector switch S,. The operation mode selector switch S,, the pulse operation mode selector switches S, and S, actuated
NANOSECOND
by a step switch, and the single pulse generating switch S4 are actuated by Plexiglas rods and magnets located at the base of the accelerator. Switches S, to S4 are operated from the control desk of the accelerator. In order to keep the reflections at the 50 ohm terminating resistor small it is important that the capacity between cathode and control electrode be small. Furthermore, the time constant of this capacity and of the internal resistor of the pulse generator determines the rise and decay time of the pulses at the cathode. Especially, the capacity between the secondary winding of the filament transformer and local ground interferes with the pulse shape. Also parasitic pulses caused by reflections at the end of the two-wire line between cathode and filament transformer distort the pulses applied to the cathode. These unwanted effects can be eliminated by winding the two-wire cathode lead around a ferrite rod to a single-layer coil. This coil together with the chassis, at control electrode potential, forms a line of high characteristic impedance. Thereby, the capacitance between cathode and control electrode of the accelerator tube has been reduced from 230 pF to about 60 pF. Such a line of high impedance connected in series with cathode and filament transformI
I
PULSE
133
DEVICE
er represents an efficient attenuation unit for disturbing pulses. The end of the line is terminated with a matched resistor, Rz6, across the capacitors, Cs and C9. Thus practically no reflected pulses come back to the cathode. The pulses at the cathode have rise and decay times of about 1.8 nsec. This time corresponds to the time constant calculated from cathode capacity and internal resistance of the pulse generator. The amplitude of the pulses at the cathode may be varied between -lOOV and -200V. The electron beam pulses generated by means of this pulse generator strike an aluminium target formed as a Faraday cage for beam measurements. The target is located in a shielding box which is flanged directly to the end of the evacuated tube extension assembly of the accelerator. A 50 ohm coaxial line connects the target to the input of an oscilloscope which is terminated by a 50 ohm resistor. For displaying the pulses a Tektronix-Sampling oscilloscope type 661 with a preamplifier type 4Sl has been used. Fig. 3 shows a pulse of 10 nsec duration as well as its rise and decay times at an accelerator voltage of 2 MV and a control voltage of 140 V for maximum cathode heating power (1.36 V, 26.3 A). The rise time is about 1 nsec, the decay time
I
DELAY ‘ERCURY -
S,
*DC PULSE
--J Fig. 2. Circuit diagram of the pulse generator.
SWITCH /
LINES
LINE
.ZL~SlU2
COILED
ON A FERRITE
-ROD
0. DEHOUST
et al.
Fig. 3. Oscilloscope trace of a 10 nsec wide beam current pulse measured at the target; accelerator voltage 2 MV, control voltage 140 V and maximum cathode heating power (1.36 V, 26.3 A). a) Entire pulse; vertical deflection factor: 1 mA/div. equivalent sweep rate: 2 nsec/div. b) Rise of the pulse; vertical de&&ion factor: 1 mA/div., equivalent sweep rate: 1 nsec/div. c) Decay of the pulse; vertical deflection factor: 1 mA/div., equivalent sweep rate: 1 nsec/div.
b
use measured at the target; accelerator voltage 1.6 MV, control V, 24.9 A). a) Entire pulse; vertical deflection factor: 0.4 mA/div., ical deflection factor: 0.4 mA/div., equivalent sweep rate: 1 nsec/div.
NANOSECOND
about 1.2 nsec. For an increase of the control voltage to 238 V without change of the other adjustments, a rise time of about 0.7 x lo-’ set and a decay time of about 1 x lo-’ set have been measured. It must be admitted that, with increasing control voltage, the initial overshoot of the pulse gets slightly higher. By reduction of the cathode heating power of the accelerator the amplitude of the pulses decreases, while, at the same time, the initial peak increases. Rise and decay times are practically independent of the pulse beam current. With decreasing accelerator voltage the rise time for small control voltage becomes shorter, while the decay time increases. At an accelerator voltage of 750 kV a rise time of about 0.7 x lo-’ set and a decay time of about 1.5 x lo-’ set were obtained for a control voltage of 140 V. In fig. 4 an electron pulse of 200 nsec duration as well as its decay characteristic are shown at an accelerator voltage of 1.6 MV, at less than full heating power, and at a control voltage of 140 V. At maximum heating current the amplitude of a beam pulse of this duration decays nearly exponentially with time. For the beam pulse in fig. 4, the cathode heating power (1.18 V, 24.9 A), and the beam pulse current (1.2 mA) accordingly, have been chosen in such a manner that approximately a square-topped pulse resulted. 3. Discussion The rise and decay times of the electron beam pulses at the target are shorter than the control pulses at the cathode because only a portion of the control pulse amplitude is required to switch the accelerator tube from cut-off to the current emitting region. The difference between rise and decay times depends on the nonlinear characteristics of rise and decay of the generator pulses at the cathode. The slope characteristics are steeper in their first portion, caused probably by mismatch at the transition points.
PULSE
DEVICE
135
In order to get a good square-topped pulse, the control pulse amplitude for short pulses should not be made much larger than the bias between cathode and control electrode is. Otherwise, the overshoot in the beginning of the beam pulse becomes higher. The overshoot is presumably caused by space charge effects between cathode and control electrode which influence the electron current from cathode to control electrode. At maximum cathode heating power a maximum recorded beam pulse current of about 5 mA at 2 MVaccelerator voltage with a control pulse amplitude of 140 V can be delivered for a short time only (in the order of 10m8 set). For longer pulses the charge travelling through the accelerator tube becomes the order of the charge stored at the cathode. For this reason, the beam pulse current decreases, more rapidly for small control pulse amplitude, as experiments confirm. Therefore, in order to obtain the maximum beam current for long pulses of square-topped shape, the control pulse amplitude should be made as high as possible. The authors wish to thank Professor M. Knoll for his kind support of this work. One of them (E.H.) is greatly indebted to the Alexander-von-HumboldtStiftung for the award of a maintenance grant. The technical assistance of Messrs. M. Gross and K. Kreuzer is also gratefully acknowledged. References 1)G. K. Wertheim and W. M. Augustyniak,
Rev. Sci. Instr. 27 (1956) 1062. 2) R. Bauerlein, Z. angew. Physik 14 (1962) 408. 3) W. Gentner, H. Maier-Leibnitz and W. Bothe, An atlas of typical expansion chamber photographs (Pergamon Press, London, 1954) p. 55. 4) R. J. Connor, Nucl. Instr. and Meth. 11 (1961) 122. 5) I. A. D. Lewis and F. H. Wells, Millimicrosecond pulse techniques (2nd ed., Pergamon Press, London, 1959).
INSTRUMENTS
NANOSECOND
AND
METHODS
40 (1966)131-135;0 NORTH-HOLLAND
PULSE DEVICE FOR A 2 MeV VAN DE GRAAPF ELECTRON O.DEHOUST,H.BAIERL*
PUBLISHING
co.
ACCELERATOR
andE.HESSE+
Znstitut fiir Technische Elektronik der Technischen Hochschule, M&hen,
Germany
Received 15 September 1965
A pulse generator for a 2 MeV Van de Graaff accelerator is described which produces electron pulses with rise and decay times of lo-9 sec. The system is capable of delivering a maximum current of about 5 mA for a duration in the order of lo-8 sec.
For longer pulses the current pulse amplitude must be reduced, for example to 1.2 mA for a 2 x 10-7 set wide pulse, in order to obtain a good square-topped shape.
1. Introduction
homogeneous ionization can be produced by 2 MeV electrons is about 2.5 mm for silicon3).
Measurements of short carrier lifetimes in semiconductors and fast decay times in scintillation materials require a source of excitation with a switching time in the order of a nanosecond. Flashes of light with the necessary time resolution can be produced by use of a rotating mirror or a Kerr cell. The intensity of such flashes of light, however, is low and insufficient for measurement of very short lifetimes. Also the penetration depth of light is small when absorption takes place in base lattice which is necessary for a strong excitation. A more suitable excitation source is a pulsed electron beam. Wertheim and Augustyniaki) produced electron pulses with rise and decay times in the order of 5 x 10m9 set by pulsing a horizontal Van de Graaff accelerator (1 MV, 25 PA). Bauerlein2) obtained switching times shorter than 10m9 set by using a pulsed electron source in a 500 keV after-accelerator in Greinacher-circuit. In this paper a pulse device for a 2 MeV Van de Graaff accelerator is described which produces electron pulses with rise and decay times in the order of 10m9sec. This device may be located in every Van de Graaff accelerator without essential changes of the accelerator arrangement, provided there is enough room. In case higher energetic particles are used, the rate of change in the structure of the specimen under examination will increase, certainly, but no noticeable changes will occur within the normal time of experiment, provided a sufficiently low pulse repetition rate and beam current will be chosen. The advantage of applying particles of higher energy is that a homogeneous ionization may be produced even in thick samples. If the excitation is considered to be of sufficient homogeneity within 3 of the particle range, the maximum thickness in which a * Present address: Brown, Boveri and Cie AG, Marmheim, Germany. + Alexander von Humboldt Fellow. Present address: University of Chile, Santiago, Chile.
2. Apparatus There are three different methods for pulsing the electron beam of an electrostatic accelerator4) : 1. Pulsing the electron source by variation of the voltage between cathode and a control electrode. 2. Electric or magnetic deflection of the electron beam across an aperture before acceleration. 3. Sweeping the beam across an aperture after the electrons have passed the accelerating voltage. Furthermore, it is possible to combine two methods, for example methods 1 and 3. For the device described in this paper the first method was chosen, because it may be applied without important change in the system arrangement, allowing peak pulse currents much greater than the dc rating of the machinel,‘). In addition, there is no scattered electron and X-ray background during the off-period. The accelerator used is a 2 MeV Van de Graaff of the High Voltage Engineering Corporation in vertical arrangement. The accelerator tube allows a maximum beam current of 250 PA in dooperation. It contains a directly heated cathode enclosed in a cylindrical cap with an aperture for the electron beam. The beam current of the tube can be varied by changing the potential difference between cathode and cathode cap. The control characteristics of the accelerator tube for different accelerating voltages during normal Van de Graaff operation are shown in fig. 1. These characteristics were obtained with a first column resistor of 1430 megohm which is located between terminal plate (potential of the cathode cap) and first accelerator-tube electrode and which is equal to the second column resistor. Since the electrostatic field of the first accelerator electrode extends into the cathode region and withdraws emitted electrons the control characteristics of the accelerator tube depend on the magnitude of the first column resistor. The necessary variation of the bias 131
132
0. DEHOUST
CONTROL Fig. 1. Control characteristics
et al.
VOLTAGE
U, IN
VOLTS
of the accelerator tube for different accelerator voltages.
between cathode cap and cathode during measurement of the characteristics was performed by a potentiometer located for this purpose in the high voltage terminal. Hereto, the movement of a step switch operated by an insulated rod was coupled to the axis of the potentiometer by means of a transmission gearing. The tappets of the potentiometer have been removed so that the potentiometer axis may be turned in one direction as many times as wanted. Since a 360 rotation of the potentiometer does not coincide with an integral multiple of a switch step, the potentiometer adjustments after a full rotation are not the same as those before rotation. This makes clear why a L- 10% error mentioned in fig. 1 is caused. To cut off the electron beam for all accelerator voltages the cathode is biased +80 V with respect to the control electrode (grid), which is maintained at local ground potential (i.e. terminal voltage). Short negative square-topped pulses with an amplitude of more than - 80 V cancel the gate voltage and cause an electron current from the cathode to the target for the duration of the impulse. A pulse generator furnished with delay line supplies the negative pulses with rise and decay times less than 2 x low9 set (measured between 10% and 90% amplitude levels). A coaxial line with open end (Amphenol: type RG/U9B) is charged up to a potential U,. By a relay with mercury wetted contacts (Clare: type HG) a 50 ohm resistor matched to the characteristic impe-
dance of the line is put across the input end of the charged line. At this terminating resistor a pulse is produced with an amplitude of +U, and a duration of 22, where z is the time a wave needs for propagation along the length of the line5). The pulse duration can be varied by changing the length of the line. Three cables of different lengths placed on a cable reel in the terminal of the accelerator provided pulses with durations of 10, 50 or 200 nsec. Relay and terminating resistor are mounted in a coaxial housing in order to prevent pulse reflections. Fig. 2 shows details of the pulse generator. The relay of the mercury switch is energized by a transistorized monostable multivibrator. The monovibrator can be triggered by a single pulse induced by the switch S4 or by periodical pulses generated by an astable multivibrator. The pulse repetition rate may be altered by the potentiometer R4 in the range between 10 and 50 cps. It may be increased to 300 cps by insignificant changes of the multivibrator and by use of another Clare-relay (type HGS). The power supply unit provides the operating voltage for the transistorized multivibrators, the cut-off voltage applied between cathode and control electrode, and the negative voltage for charging the delay line. The charging voltage and, therefore, the amplitude of the pulse, generating the beam current, may be altered by the selector switch S,. The operation mode selector switch S,, the pulse operation mode selector switches S, and S, actuated
NANOSECOND
by a step switch, and the single pulse generating switch S4 are actuated by Plexiglas rods and magnets located at the base of the accelerator. Switches S, to S4 are operated from the control desk of the accelerator. In order to keep the reflections at the 50 ohm terminating resistor small it is important that the capacity between cathode and control electrode be small. Furthermore, the time constant of this capacity and of the internal resistor of the pulse generator determines the rise and decay time of the pulses at the cathode. Especially, the capacity between the secondary winding of the filament transformer and local ground interferes with the pulse shape. Also parasitic pulses caused by reflections at the end of the two-wire line between cathode and filament transformer distort the pulses applied to the cathode. These unwanted effects can be eliminated by winding the two-wire cathode lead around a ferrite rod to a single-layer coil. This coil together with the chassis, at control electrode potential, forms a line of high characteristic impedance. Thereby, the capacitance between cathode and control electrode of the accelerator tube has been reduced from 230 pF to about 60 pF. Such a line of high impedance connected in series with cathode and filament transformI
I
PULSE
133
DEVICE
er represents an efficient attenuation unit for disturbing pulses. The end of the line is terminated with a matched resistor, Rz6, across the capacitors, Cs and C9. Thus practically no reflected pulses come back to the cathode. The pulses at the cathode have rise and decay times of about 1.8 nsec. This time corresponds to the time constant calculated from cathode capacity and internal resistance of the pulse generator. The amplitude of the pulses at the cathode may be varied between -lOOV and -200V. The electron beam pulses generated by means of this pulse generator strike an aluminium target formed as a Faraday cage for beam measurements. The target is located in a shielding box which is flanged directly to the end of the evacuated tube extension assembly of the accelerator. A 50 ohm coaxial line connects the target to the input of an oscilloscope which is terminated by a 50 ohm resistor. For displaying the pulses a Tektronix-Sampling oscilloscope type 661 with a preamplifier type 4Sl has been used. Fig. 3 shows a pulse of 10 nsec duration as well as its rise and decay times at an accelerator voltage of 2 MV and a control voltage of 140 V for maximum cathode heating power (1.36 V, 26.3 A). The rise time is about 1 nsec, the decay time
I
DELAY ‘ERCURY -
S,
*DC PULSE
--J Fig. 2. Circuit diagram of the pulse generator.
SWITCH /
LINES
LINE
.ZL~SlU2
COILED
ON A FERRITE
-ROD
0. DEHOUST
et al.
Fig. 3. Oscilloscope trace of a 10 nsec wide beam current pulse measured at the target; accelerator voltage 2 MV, control voltage 140 V and maximum cathode heating power (1.36 V, 26.3 A). a) Entire pulse; vertical deflection factor: 1 mA/div. equivalent sweep rate: 2 nsec/div. b) Rise of the pulse; vertical de&&ion factor: 1 mA/div., equivalent sweep rate: 1 nsec/div. c) Decay of the pulse; vertical deflection factor: 1 mA/div., equivalent sweep rate: 1 nsec/div.
b
use measured at the target; accelerator voltage 1.6 MV, control V, 24.9 A). a) Entire pulse; vertical deflection factor: 0.4 mA/div., ical deflection factor: 0.4 mA/div., equivalent sweep rate: 1 nsec/div.
NANOSECOND
about 1.2 nsec. For an increase of the control voltage to 238 V without change of the other adjustments, a rise time of about 0.7 x lo-’ set and a decay time of about 1 x lo-’ set have been measured. It must be admitted that, with increasing control voltage, the initial overshoot of the pulse gets slightly higher. By reduction of the cathode heating power of the accelerator the amplitude of the pulses decreases, while, at the same time, the initial peak increases. Rise and decay times are practically independent of the pulse beam current. With decreasing accelerator voltage the rise time for small control voltage becomes shorter, while the decay time increases. At an accelerator voltage of 750 kV a rise time of about 0.7 x lo-’ set and a decay time of about 1.5 x lo-’ set were obtained for a control voltage of 140 V. In fig. 4 an electron pulse of 200 nsec duration as well as its decay characteristic are shown at an accelerator voltage of 1.6 MV, at less than full heating power, and at a control voltage of 140 V. At maximum heating current the amplitude of a beam pulse of this duration decays nearly exponentially with time. For the beam pulse in fig. 4, the cathode heating power (1.18 V, 24.9 A), and the beam pulse current (1.2 mA) accordingly, have been chosen in such a manner that approximately a square-topped pulse resulted. 3. Discussion The rise and decay times of the electron beam pulses at the target are shorter than the control pulses at the cathode because only a portion of the control pulse amplitude is required to switch the accelerator tube from cut-off to the current emitting region. The difference between rise and decay times depends on the nonlinear characteristics of rise and decay of the generator pulses at the cathode. The slope characteristics are steeper in their first portion, caused probably by mismatch at the transition points.
PULSE
DEVICE
135
In order to get a good square-topped pulse, the control pulse amplitude for short pulses should not be made much larger than the bias between cathode and control electrode is. Otherwise, the overshoot in the beginning of the beam pulse becomes higher. The overshoot is presumably caused by space charge effects between cathode and control electrode which influence the electron current from cathode to control electrode. At maximum cathode heating power a maximum recorded beam pulse current of about 5 mA at 2 MVaccelerator voltage with a control pulse amplitude of 140 V can be delivered for a short time only (in the order of 10m8 set). For longer pulses the charge travelling through the accelerator tube becomes the order of the charge stored at the cathode. For this reason, the beam pulse current decreases, more rapidly for small control pulse amplitude, as experiments confirm. Therefore, in order to obtain the maximum beam current for long pulses of square-topped shape, the control pulse amplitude should be made as high as possible. The authors wish to thank Professor M. Knoll for his kind support of this work. One of them (E.H.) is greatly indebted to the Alexander-von-HumboldtStiftung for the award of a maintenance grant. The technical assistance of Messrs. M. Gross and K. Kreuzer is also gratefully acknowledged. References 1)G. K. Wertheim and W. M. Augustyniak,
Rev. Sci. Instr. 27 (1956) 1062. 2) R. Bauerlein, Z. angew. Physik 14 (1962) 408. 3) W. Gentner, H. Maier-Leibnitz and W. Bothe, An atlas of typical expansion chamber photographs (Pergamon Press, London, 1954) p. 55. 4) R. J. Connor, Nucl. Instr. and Meth. 11 (1961) 122. 5) I. A. D. Lewis and F. H. Wells, Millimicrosecond pulse techniques (2nd ed., Pergamon Press, London, 1959).